1. Field of the Invention
The invention relates generally to storage systems. More specifically, the invention relates to making a replica of a snapshot image in a remote network attached storage (NAS) system, and to enabling a remote NAS system to mount the snapshot image after making the replica of the snapshot image.
2. Description of the Related Art
Generally speaking, there are two major storage system types: SAN (Storage Area Network) and NAS (Network Attached Storage). SAN employs block level access to data in its storage system, while file level access is used in NAS.
“Snapshot” is a mechanism for taking a point in time (PIT) copy of a file system in the NAS system. There are various ways of implementing the snapshot mechanism. Copy on write (COW) is one of the most popular implementations in the NAS system.
“Remote Copy” is a mechanism for creating a volume pair between two separate storage systems. One main purpose of remote copy is to prepare for catastrophic events or disasters, and to enable disaster recovery (DR). Although the NAS system is a file level storage system, it can collaborate with the remote copy mechanism in a disk array storage system residing in the backend of the NAS system.
It is possible to use both the snapshot mechanism and the remote copy mechanism at the same time. More specifically, the snapshot image can be copied by using remote copy operations. However, in the prior art, it is not possible for a local file system of a remote NAS system to mount the copied snapshot image because the snapshot module in the remote NAS system does not have sufficient information regarding logical units corresponding the snapshot image.
Examples of prior art patents include: “Method and System for Providing a Static Snapshot of Data Stored on a Mass Storage System”, Richard S. Ohrann et al, U.S. Pat. No. 5,649,152; “Storage System Assuring Data Integrity and A Synchronous Remote Data Duplexing”, Akira Yamamoto, U.S. Pat. No. 6,408,370; and “Heterogeneous Computer System, Heterogeneous Input Output System and Data Backup Method for the Systems”, Yasuko Fukuzawa et al, U.S. Pat. No. 6,529,976. The entire disclosures of all of these patents are hereby incorporated by reference.